1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image on a paper by visualizing an electrostatic latent image formed on a photosensitive drum by supplying a developer and by transferring this visualized developer image on the paper.
2. Description of the Related Art
As disclosed in Japanese Patent Publication (Kokoku) No. 61-32667, in a conventional copying machine, at the same time when transfer current is given to the transfer charger, the voltage of the same polarity as the voltage applied to the transfer charger is applied to a transfer guide which leads a paper to the transfer position, thus, the undesired outflow of the charge given by the transfer charger is prevented.
By the way, the transfer current that is given to the transfer charger is given immediately before the leading edge of the paper comes near the transfer position in order to obtain good transfer efficiency. It was found that pits are produced by the excessive transfer in an area near the leading edge of the paper if the transfer guide bias was applied at the same timing as this transfer current.
That is, in the state before the leading edge of the paper being conveyed along the transfer guide, of which electric resistance is dropped, comes near the separation charger positioned at the downstream side of the transfer charger, the electric charge given by the transfer charger is considered to flow out via the transfer guide only. Further, it is also considered that the electric charge given by the transfer charger flows to the transfer guide and also to the separation charger when the leading edge of the paper comes near the separation charger and the outflow amount of the charge increases. Accordingly, if this transfer guide bias is set at a size to give a proper potential at the central part occupying the greater part of the paper, the potential near the leading edge of the transfer paper before it comes near the separation charger becomes large unnecessarily and such transfer faults as pits due to excessive transfer are produced.
As a countermeasure to prevent such transfer faults, the control is considered to change a transfer current value that is given to the transfer charger before and after the leading edge of the paper comes near the separation charger but there is caused a new problem that such transfer faults as pits due to insufficient transfer are produced near the leading edge of the paper if a normal paper of which electric resistance is not dropped is supplied.